1. Field of the Invention
This invention relates to a marine propulsion unit for a watercraft, and more particularly to a tilt and trim unit for a marine propulsion unit.
2. Description of the Related Art
Outboard motors with four-cycle engines have grown in popularity in recent years, due in part to environmental concerns associated with two-cycle outboard motors. The application of four-cycle engines in outboard motors, however, has raised some challenges, especially with large horse power engines. A four-cycle engine will weigh more than a two-cycle engine that produces a comparable horsepower to that of the four-cycle engine. The additional weight creates problems for the conventional hydraulic tilt and trim units used with the outboard motor.
A tilt and trim unit commonly operates between components of a clamping assembly to adjust the trim and tilt position of the outboard motor. In particular, the tilt and trim unit usually includes an extendable hydraulic cylinder, piston assembly that operates between a clamping bracket and a swivel bracket of the clamping assembly, which typically supports an outboard motor on a watercraft. The clamping bracket is attached to the watercraft and the swivel bracket supports the outboard motor. A pivot pin connects together the swivel and clamping brackets. Extension of a rod of the cylinder causes the swivel bracket to pivot about the axis of the pivot pin, relative to the stationary clamping bracket, to raise or lower the outboard drive. This assembly prevents the outboard motor from popping up when operating in reverse, while permitting the outboard motor to pop up when it strikes an underwater object as it travels forward.
Manually operated tilt and trim units usually include a bypass passage that interconnects chambers of the cylinder which the piston separates. A valve assembly is located within the passage to regulate flow through the passage. When the valve assembly is opened, the outboard motor can be manually tilted up without having to act against the resistance of the hydraulic cylinder. U.S. Pat. No. 4,784,625 entitled "Tilt Lock Mechanism For Marine Propulsion Device," illustrates an exemplary arrangement of the valve assembly and bypass passage of a tilt and trim unit. The tilt lock mechanism disclosed in the '625 patent includes a separate accumulator chamber that communicates with the two chambers of the hydraulic cylinder. Check valves normally restrict flow between the cylinder chambers and the accumulator chamber. An actuating mechanism selectively opens one of the check valves or the other to permit manual movement of the outboard motor, either up or down, without working against the hydraulic cylinder.
FIG. 1 illustrates another prior construction of a hydraulic cylinder, piston assembly 20, in cross-section. The prior cylinder 20 includes a piston 22 that slides within a bore 24 of the cylinder 20. The piston 22 divides the bore 24 into upper and lower fluid chambers 26, 28.
A piston rod 30 is attached to the piston 22 and extends beyond one end of the cylinder 20, through the upper chamber 26. The outer end of the piston rod 30 includes a trunnion 32 that is pivotally attached to an associated swivel bracket. The cylinder body 20 also includes a lower trunnion 34 that is pivotally connected to the associated clamping bracket.
A passage 36 extends generally parallel and next to the cylinder bore 24, on one side of the cylinder 20, and connects together the upper and lower fluid chambers 26, 28. A manual valve 38 operates within the passage 36 to control the flow of working fluid through the passage 36. When the valve 38 is open, the upper and lower fluid chambers 26, 28 communicate with each other through the passage 36, and the piston 22 can be easily slid within the bore 24. The outboard motor thus can be raised and lowered, unencumbered by the hydraulic cylinder assembly 20. When the valve 38 is closed, however, the hydraulic cylinder assembly 20 locks the outboard motor in the established tilt or trim position. An actuator 39 is used to open and close the valve 38.
As seen in FIG. 1, the cylinder assembly 20 also includes an accumulation chamber 40 arranged on one side of the cylinder bore 24. The accumulation chamber 40 communicates with the lower fluid chamber 28 of the cylinder 20 to compensate for the volumetric differences between the upper and lower fluid chambers 26, 28. That is, because the piston rod 30 extends through the upper chamber 26, and thus reduces the volume in the upper chamber 26, less fluid will be displaced from the upper chamber 26 than is required to make up the volume in the lower chamber 28 as the piston 22 moves upwardly. The accumulation chamber 40 directly communicates with the lower fluid chamber 28 to compensate for this volumetric difference between the fluid chambers 26, 28. The accumulation chamber 40 is arranged on a side of the cylinder bore 24 opposite of the passage 36 and the manual valve 38.
Another prior cylinder assembly for a tilt locking mechanism is illustrated in U.S. Pat. No. 5,368,509, issued Nov. 29, 1994, and entitled "Tilt Lock System For Outboard Motor." The construction of this assembly is generally similar to the cylinder assembly construction illustrated in FIG. 1, but with a multi-position valve located on the same side of the cylinder as the accumulation chamber and interposed between these components of the cylinder assembly.
The constructions of both tilt and trim units, illustrated in FIG. 1 and disclosed in the '509 patent, result in a wide assembly. The swivel and clamping consequently must be wide and reinforced to accommodate the hydraulic cylinder assembly. The increased weight of the new four-cycle outboard motors further exacerbates this problem, requiring additional reinforcement of the brackets. Such reinforcing increases the size and weight of the brackets, as well as increases the manufacturing cost.
An additional prior construction of a hydraulic cylinder assembly 50 is illustrated in FIG. 2, which depicts the cylinder assembly in cross-section. In this cylinder assembly 50, an accumulation chamber 52 is integrated into an upper fluid chamber 54 above a port 56 that communicates with the upper fluid chamber 54. FIG. 2 illustrates the position of a piston 58 and piston rod 60 in a fully retracted position. Under this condition, a volume of working fluid F1 remains above the piston 58 with a volume of inert compressible gas F2 residing above the working fluid F1 to form the accumulation chamber 52 above the upper fluid chamber 54.
The width of the tilt and trim unit illustrated in FIG. 2 is less than the width of the unit illustrated in FIG. 1; however, the length of the unit increases as a result of the location of the accumulation chamber above the piston. In order for the unit to fit between the clamping and swivel brackets with the outboard motor in a fully trimmed down position, the size of the accumulation chamber must be reduced. A smaller accumulation chamber consequently reduces the diameter size of the piston. And a smaller size piston reduces the amount of weight the hydraulic unit can support so that the unit cannot be used with heavy outboard motors.
In addition, both the cylinder assembly designs illustrated in FIGS. 1 and 2 are difficult to adjust, especially when supporting a heavy motor. In both prior designs, the manual valve is opened to tilt up the outboard motor. If the person adjusting the trim position wants to close the valve once the outboard motor has been raised to the desired position, the user cannot simply let go of the outboard motor as it will immediately lower (i.e., trim or tilt down) under its own weight. The person thus must hold the heavy outboard motor while leaning over the transom of the watercraft to close the valve. This operation is difficult and awkward for one person to perform alone.